1. Field of the Invention
The present invention relates to a control system and method for a vehicle with a first powertrain including an engine and a second powertrain including an electric motor powered by an engine driven generator.
2. Description of the Background Art
Vehicles are proposed, which have a first powertrain including an engine and an automatic transmission, and a second powertrain including an electric motor powered by a generator driven by the engine. The first powertrain is coupled with a first set of road wheels. The second powertrain is coupled with a second set of road wheels. Several examples of such vehicles are available.
One such example is disclosed in Japanese Utility Model Application Pre-Grant Publication No. S55-110328, published Aug. 2, 1980. In this example, an internal combustion engine is coupled with a set of front road wheels via a transmission and a front differential gear to define a first powertrain. An electric motor is constantly coupled with a set of rear road wheels via a rear differential gear to define a second powertrain. The motor is energized to produce torque only when electric power is supplied from a generator driven by the engine. A controller is provided to regulate field current passing through the generator. The controller includes a differential amplifier. The differential amplifier receives first and second input voltage signals. The first voltage signal is indicative of wheel speed of the front road wheels. The second voltage signal is indicative of wheel speed of the rear road wheels. The differential amplifier has its output connected with a base of a transistor. The transistor has its collector connected with one terminal of field winding of the generator. The other terminal of the field winding is connected via a mode switch with a battery. In operation, whenever the front road wheels slip, the wheel speed of the front road wheels exceeds the wheel speed of the rear road wheels. This causes the differential amplifier to output a voltage signal, rendering the transistor conductive, allowing field current to pass through the field winding of the generator. The field current and revolution speed determine the voltage of electric power produced by the generator and supplied to the electric motor, causing the electric motor to produce torque. In this manner, whenever the front road wheels slip, the electric motor produces torque to drive the rear road wheels.
Another example is disclosed in Japanese Patent Application Pre-Grant Publication No. H07-231508, published Aug. 29, 1995. In this example, too, an internal combustion engine is coupled with a set of front road wheels via a transmission and a front differential gear to define a first powertrain. An electric motor is coupled with a set of rear road wheels via a rear differential gear to define a second powertrain. Unlike the first mentioned example, a clutch is provided in the second powertrain to interrupt connection between the electric motor and the set of rear road wheels. The provision of such clutch aims at protection of the electric motor. A generator is coupled with the engine to provide electric power to the electric motor. A microprocessor-based controller is provided. The controller determines whether or not current operating conditions correspond to predetermined operating conditions suitable for the electric motor to drive the rear road wheels. The controller receives information as to accelerator pedal position, front wheel speed and rear wheel speed. The controller determines an appropriate value of standard wheel speed for the current value of accelerator pedal position by looking into a look-up table. The look-up table contains various values of standard wheel speed indexed by different values of accelerator pedal position. The controller includes a comparator where the standard wheel speed is compared to the front wheel speed and also to the rear wheel speed. The comparator provides a deviation of the front wheel speed from the standard wheel speed, a deviation of the rear wheel speed from the standard wheel speed, and a deviation of the front wheel speed from the rear wheel speed. Each of these deviations is compared to a threshold to provide one of three different criteria, positive, zero and negative. These criteria constitute elements of a comparison data. The controller determines whether or not the comparison data correspond to any one of predetermined data that justify the electric motor operation to drive the rear road wheels. In this example, there is no description on when and how the controller generates command for engagement or disengagement of the clutch.
Still another example is disclosed in Japanese Patent Application Pre-Grant Publication No. P2000-318473A, published Nov. 21, 2000. In this example, too, an internal combustion engine is coupled with a set of front road wheels via a transmission and a front differential gear to define a first powertrain. An electric motor is coupled with a set of rear road wheels via a rear differential gear to define a second powertrain. A dog clutch is provided in the second powertrain to interrupt connection between the electric motor and the set of rear road wheels. The provision of such clutch aims at protection of the electric motor. A generator is coupled with the engine to provide electric power to the electric motor. A microprocessor-based controller is provided. The controller conducts a start-assist control mode when, with a brake pedal released and a manual select lever placed in a forward drive position, there is a slip between front and rear road wheels at vehicle speeds lower than a predetermined speed value. In this control mode, the controller generates a clutch engagement command, causing a solenoid to engage the dog clutch. The dog clutch remains engaged until the vehicle speed exceeds the predetermined speed value. To protect the motor, the controller generates a clutch disengagement command when the vehicle speed exceeds the predetermined speed value. The controller regulates field current passing through the engine driven generator to cause the motor to vary its output torque in such a manner as to reduce the slip.
Other example is disclosed in Japanese Patent Application Pre-Grant Publication No. P2002-171607A, published Jun. 14, 2002. In this example, too, an internal combustion engine is coupled with a set of front road wheels via a transmission and a front differential gear to define a first powertrain. An electric motor is coupled with a set of rear road wheels via a rear differential gear to define a second powertrain. An electromagnetic clutch is provided in the second powertrain to interrupt connection between the electric motor and the set of rear road wheels. The provision of such clutch aims at protection of the electric motor. Unlike the above-mentioned examples, a battery provides electric power to the electric motor and also to the electromagnetic clutch. A microprocessor-based controller is provided. In order to suppress vibrations of the electric motor, the controller conducts a feedback control by regulating the electric motor to reduce a deviation of a current value of motor speed and a desired value thereof toward zero. The desired value is determined based on the result from calculating a desired value of driving force. The desired value of driving force is determined as a function of accelerator pedal position and vehicle speed.
The prior art control systems are to supply an electric motor with electric power from an engine driven generator when an acceleration slip occurs between the first and second sets of road wheels. Supply of electric power causes the motor to produce torque, applying driving force in opposed relationship to road load from the second set of road wheels. If torque output of the motor fails to achieve a satisfactory level, the motor may apply running resistance to the second set of road wheels. Sufficiently high efficiency and excellent fuel economy cannot be achieved if occurrence of such running resistance is allowed.
Apparently, the inventors of the prior art proposed systems fail to address themselves to a task of solving the above-mentioned problem.
Commonly Owned U.S. Pat. No. 6,434,469 B1 (Date of Patent: Aug. 13, 2002), which has been hereby incorporated by reference in its entirety, discloses a control system for a vehicle with a first powertrain and a second powertrain. The first powertrain includes an engine and it is coupled with a first set of road wheels. The second powertrain includes an electric motor and it is coupled with a second set of road wheels. The vehicle includes a generator coupled with the engine, the generator being provided as a source of electric power for the electric motor. This previously proposed control system provides enhanced acceleration performance by adjusting load torque of the generator to acceleration slip between the first and second sets of road wheels.
Commonly Owned Co-pending U.S. patent application Ser. No. 10/237,968, which has been hereby incorporated by reference in its entirety, was filed on Sep. 9, 2002 in the name of Hideyuki SAEKI et al. for the invention entitled “VEHICLE WITH CLUTCH FOR TRANSMISSION OF TORQUE OUTPUT OF MOTOR”. This Co-pending Application claims priority on Japanese Patent Application No. 2001-274123 (Filing Date: Sep. 10, 2001) and Japanese Patent Application No. 2001-367541 (Filing Date: Nov. 30, 2001). This incorporated Co-pending U.S. application discloses a control system for a vehicle with a first powertrain and a second powertrain. The first powertrain includes an engine and it is coupled with a first set of road wheels. The second powertrain includes an electric motor and it is coupled with a second set of road wheels. The vehicle includes a generator coupled with the engine, the generator being provided as a source of electric power for the electric motor. The second powertrain includes a hydraulic or wet clutch for transmission of torque output of the electric motor to the second set of road wheels and transmission of road load from the road wheels to the electric motor. According to this control system, the hydraulic clutch is disengaged when a current vehicle speed exceeds a threshold and the threshold is altered. Accounting for clutch drag state of the hydraulic clutch, the control system alters the threshold.
The previously proposed control systems and methods are satisfactory to some extent. However, a need remains for improving the proposed control systems and methods so as to achieve enhanced efficiency and fuel economy by solving the problem that the electric motor may apply running resistance to the second set of road wheels if torque output of the motor drops.
Accordingly, an object of the present invention is to provide control system and method that have achieved enhanced efficiency and fuel economy by solving the problem that the electric motor may apply running resistance to the second set of road wheels if torque output of the motor drops.